Refractory metals high-temperature mechanical properties

Properties. Properties of SiC fibers are shown in Table 19.2. They are similar to those of CVD boron fibers except that SiC is more refractory and less reactive than boron. CVD-SiC fibers retain much of their mechanical properties when exposed to high temperature in air up to 800°C for as long as one hour as shown in Fig. 19.3. [ 1 SiC reacts with some metals such as titanium in which case a diffusion barrier is applied to the fiber (see Sec. 2.5 below). 

Of course, the basic requirement of the metal oxide applied in the thermochemical cycle for CO2 splitting is the feasibility of the thermal reduction and the reoxidation by CO2 in reasonable temperature ranges. Regardless of the impact of very high temperatures on important properties of the thermochemical cycle, this demand is also related to the availability of refractory construction materials in terms of mechanical, but also chemical stability under these conditions. 

Briefly, three points of porous SiC-based catalytic support properties can be emphasized (i) SiC shows very good mechanical properties which gives resistance to erosion and attrition, in addition to a high thermal stability (ii) SiC has a higher thermal conductivity compared with the more conventional supports which could prevent the metal sintering (iii) SiC is particularly inactive with respect to chemical reagents such as acids or bases. Therefore, the active phase can be easily reprocessed after simple acidic or basic treatments. Among refractory materials, the thermal conductivity of silicon carbide, SiC (500 W m-1K-1 for crystalline state, at room temperature) is close to that of metals such as Ag or Cu (400-500 Wm K-1). 

Tantalum Tantalum has unique properties that make it useful for many applications, from electronics to mechanical and chemical systems. Many efforts have been made to develop an electroplating process for the electrodeposition of Ta. High-temperature molten salts were found to be efficient baths for the dectrodepo-sition of refractory metals. To the best of our knowledge, imtil now no successful attempts have been made for Ta electrodeposition at room temperature or even at low temperature in ionic liquids. We present here the first results of tantalum dec-trodeposition in the air and water stable ionic liquid 1-butyl-l-methyl-pyrrolidinium bis(tri-fiuoromethylsulfonyl)amide ([BMP][Tf2N]). 

Niobium, molybdenum tantalum and tungsten are refractory metals, i.e. the ones that show resistance against attack by liquid RE metals. These metals are usually listed in the literature in order of decreasing solubility in the liquid rare metals at high temperatures. Tungsten is the least soluble. However, tungsten is rather brittle and has poor mechanical properties compared to tantalum, which is the second best with respect to solubility. Tantalum containers are therefore used in purihcation of liquid rare element metals (Gupta and Krishnamurthy 2005). 

Aluminum nitride may be used in composite structures containing aluminum for either structural or electronic applications, due to its attractive thermal, electronic, and mechanical properties [176-178]. AlN ceramics are also known to have a sufficiently high-temperature compatibility with refractory metals. Finally, AlN is an ecologically safe material. The structure of AlN as a ceramics layer of the multilayer Al/AlN composites has been investigated to only a limited degree [179]. 

The carbon-fiber/polymer composites reviewed in the previous section have excellent mechanical properties but limited temperature resistance. Maximum operating temperature is presently 370°C (Table 9.2). These composites cannot meet the increasingly exacting requirements of many aerospace applications which call for a material with low density, excellent thermal-shock resistance, high strength, and with temperature resistance as high or higher than that of refractory metals or ceramics. These requirements are met by the so-called carbon-carbon materials. 

It should be noted first that the mechanical properties of refractory metals and alloys are sensitive to minor impurities, such as O, C, and N levels, grain size and orientation, precipitates, dislocations, and other microstructural parameters. Moreover, impurity levels and microstractural states may change during high-temperature testing, which also influence the properties. Comparison of the mechanical properties is made in this section based on literature data, in which, however, details of the materials and testing information are unavailable in most cases. 

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